1. Field of the Invention
The present invention is concerned with the destructive distillation (or pyrolysis) of organic material, and particularly (but not exclusively), the pyrolysis of waste tire material or other similar rubber compounds.
2. Description of the Prior Art
A process of pyrolyzing organic waste by means of microwaves is disclosed in U.S. Pat. No. 3,843,457. In this process organic industrial and household wastes are comminuted and the comminuted solid material is subjected to a microwave discharge in a gas stream at a pressure substantially less than atmospheric for a contact time of the order of one second. The gas stream and the comminuted solid material pass through the microwave zone in parallel, with a view to maximizing interaction between the gas and the solid.
Microwave destruction of waste tire material has also been disclosed in, for example, (examined) Japanese Patent Applications 76/47192-3, 77/26553, 82/3491 and (unexamined) Japanese patent applications 52/3672 and 51/123287. These documents describe laboratory scale processes, in which small quantities of the waste rubber are subjected to microwave discharge so as to completely break the rubber down and produce carbon black and hydrocarbons (gaseous or liquid). These documents give no details as to how the basic idea of microwave pyrolysis could be converted into a commercial process.
A more recent document, U.S. Pat. No. 4,647,443, discloses a process of pyrolyzing of waste tire material which involves subjecting the tire material to radiant heating so as to produce a pyrolyzed char, and then subjecting the char to microwave discharge to remove volatile hydrocarbons from the char, so as to produce carbon black.
The disadvantage of this process is that radiant heating of tire material involves relatively inefficient heat transfer. I have found that, contrary to the teaching of U.S. Pat. No. 4,647,443, it is advantageous to pre-heat tire material (and other organic materials), without pyrolysis thereof, prior to complete pyrolysis by means of microwaves. Microwave-induced pyrolysis has been found to be impaired if tire material (or other organic material) has previously been partly pyrolyzed (this is postulated to be because of the insulating effect of the elemental carbon produced on pyrolysis). It is believed that this may be why the U.S. patent teaches that the pyrolysis should be taken to substantial completion in the radiant heating stage.
I have now found that, in contradistinction to the teaching of U.S. Pat. No. 4,647,443, it is advantageous to completely avoid pyrolysis in a pre-heating stage prior to microwave-induced pyrolysis.
The process according to the invention enables microwave pyrolysis of waste tire material and other rubber compounds, as well as other organic materials, to be operated as a commercial destructive distillation process.